In one type of test apparatus for simulating variable air pressures for analyzing the operation of internal combustion engines, a canister, having a variable throttle valve device, is connected to receive the exhaust gas of the internal combustion engine. The combustion air of the internal combustion engine is supplied by way of an intake pipe directly from the atmosphere. A regulating valve is inserted in this intake pipe which is connected with the canister by way of a control line. The canister is provided with a valve for regulating the air being drawn into the tank which, together with the exhaust gas, is removed from the tank by a pump. Depending upon the position of the regulating valve, the vacuum in the canister may be altered, in order to establish, by way of the control line, that a vacuum exists at the regulating valve, so that suction intake operation of the engine occurs under the same conditions as the expulsion of the combustion gases.
In this device, however, it is especially disadvantageous in that the vacuum within the canister, once established by the regulating valve, continually changes during intermittent operation of the engine as a result of continuous changes in the exhaust gas flow. The intake air of the internal combustion engine regulated in accordance with the regulating in accordance with the regulating valve, must always be matched to the changing exhaust-gas flow changes. This is possible only by way of frequent regulation, so that the operating liability of such a test apparatus is considerably reduced. For a more detailed explanation of this apparatus, see German Patent Publication 317052.